Empirical age spectra for the lower tropical stratosphere from in situ observations of CO₂: Implications for stratospheric transport

Abstract

Abstract. Empirical age spectra for the lower tropical stratosphere (from the tropopause to --19.5 km) have been derived from in situ measurements of CO2, using information provided by the vertical propagation of the tropospheric seasonal cycle and long-term positive trend. Our method provides accurate and unambiguous mean ages for this region which are difficult to obtain by simple analysis of lag times from tracer measurements. We find that the air is 30-40% younger in northern spring than in autumn. For example, at 460 K the mean age (relative to the tropical tropopause) was 0.4 years in March and 0.6 years in September. The phase lag times and attenuation of CO2 seasonal extrema in the stratosphere are shown to depend on seasonal variations in transport rates and on the presence of harmonics in the CO2 boundary condition with frequencies higher than 2n/yr. Profiles of stratospheric water vapor, generated from the derived age spectra with a stratospheric boundary condition based on observed tropical tropopause temperatures, are consistent with in situ observations of H20.Comparison of the predicted water vapor seasonal cycle with satellite observations suggests that satellite-borne instruments underestimate the amplitude near the tropical tropopause. We relate the empirical age spectra to the analytic solution for the 1-D advection-diffusion tracer continuity equation to obtain seasonally resolved estimates of the ascent rate and the vertical diffusion coefficient. The derived age spectra provide a unique observation-based diagnostic for evaluating the simulation of tracer transport in models.

Simulations of preindustrial, present-day, and 2100 conditions in the NASA GISS composition and climate model G-PUCCINI

“…This is presented in Fig. 1, which shows the age of air in the stratosphere calculated using the linearly increasing tracer in comparison with values derived from SF 6 and CO 2 observations (Andrews et al, 1999;Boering et al, 1996;Elkins et al, 1996;Harnisch et al, 1996) and values from other models (Hall et al, 1999). Clearly the model's air in the high latitude lower stratosphere is too young, a feature seen in most GCMs and even in CTMs driven by assimilated meteorological data (Scheele et al, 2005;Schoeberl et al, 2003).…”

A near‐field tool for simulating the upstream influence of atmospheric observations: The Stochastic Time‐Inverted Lagrangian Transport (STILT) model

“…We ran STILT in the backward time mode to interpret CO 2 observations during the 1999 COBRA test flights, as part of which the University of North Dakota Cessna Citation aircraft acquired a time series of vertical profiles in the vicinity of the WLEF tall tower on 8 June (Figure 6). The airborne CO 2 instrument used in COBRA is a nondispersive infrared gas analyzer based on the design of the Harvard University ER-2 CO 2 analyzer [Andrews et al, 1999;Boering et al, 1994]. In-flight and laboratory calibrations indicate that the uncertainty of the CO 2 observations during COBRA was ±0.25 ppm [Daube et al, 2002].…”

Model evaluation of CO2 and SF6 in the extratropical UT/LS region

“…Even the averaged amplitude of about ±3 ppmv in the tropical lower troposphere is twice as large as the yearly growth rate. Thus, the tropospheric seasonal cycle is a dominant appearance which propagates upwards through the tropopause into the LS and spreads out merdionally, as shown by, e.g., Boering et al [1994Boering et al [ , 1996; ST98 and Andrews et al [1999Andrews et al [ , 2001a.…”